#include <stdio.h>
#include <stdlib.h>

typedef struct ListNode
{
    int val;
    struct ListNode* next;
}ListNode;

struct ListNode* detectCycle(struct ListNode* head) {
    struct ListNode* slow = head, * fast = head;
    while (fast && fast->next)
    {
        slow = slow->next;
        fast = fast->next->next;

        if (slow == fast)
        {
            struct ListNode* meet = slow;
            while (head != meet)
            {
                head = head->next;
                meet = meet->next;
            }
            return meet;
        }
    }
    return NULL;
}


int main()
{
    struct ListNode* n1 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n2 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n3 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n4 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n5 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n6 = (struct ListNode*)malloc(sizeof(struct ListNode));

    if (n1 == NULL || n2 == NULL || n3 == NULL || n4 == NULL || n5 == NULL || n6 == NULL)
    {
        perror("malloc");
        return -1;
    }

    struct ListNode* head = n1;

    n1->val = 1;
    n2->val = 2;
    n3->val = 3;
    n4->val = 4;
    n5->val = 5;
    n6->val = 6;

    n1->next = n2;
    n2->next = n3;
    n3->next = n4;
    n4->next = n5;
    n5->next = n6;
    n6->next = n4;

    ListNode* ret = detectCycle(head);
    printf("%d", ret->val);

    return 0;
}